The present invention relates generally to DC powered hand held tools and electronic controls for direct current motors and more particularly to a new and improved dynamic braking circuit for electronically commutated (brushless) direct current hand held tools.
Electronic commutation in direct current (DC) motors is well documented. Such electronic commutation eliminates the brushes and commutators long associated with DC motors along with the associated friction, brush wear, generation of contamination particles and arcing. In surgical tools, these commutators and associated brushes and hardware are adversely affected by steam autoclaving (sterilization) often used with such tools.
Hand held rechargeable battery powered tools are also commonplace including electric drills, screwdrivers, grass trimmers and the like. Most of these tools rely on friction produced largely by the brushes to slow and stop the bit or blades when a control trigger is released.
In U.S. Pat. No. 5,235,261 there is disclosed a DC powered surgical handpiece which includes a circuit for braking the motor to a halt when the operator releases a control trigger. Braking in this patented arrangement is achieved by energizing all three winding switches (transistors) when a zero speed command is sensed. Since this patented arrangement requires a half wave design, this also shorts the battery to ground through the winding wasting battery power, and creating additional stress on the switches (transistors).
With such an arrangement, considerable battery power is consumed reducing the operating time available to a surgeon, and reducing battery life. Moreover, with the patented arrangement initial braking is delayed until the speed command signal has dropped to zero, and due to winding inductance created by the pulsing of the brake circuit, the retarding circuit reduces the braking effect at high speeds over what would otherwise be available. Braking with the patented circuit is available least, when needed most. Also due to the pulsing of the brake signal, reverse rotation can occur as the motor comes to a halt. The patented structure is highly complex with many components such as a step up switching power supply and other components likely to compromise reliability when subject to steam autoclaving.
It is therefore highly desirable to provide a new and improved DC powered hand held tool.
It is also highly desirable to provide a new and improved DC powered surgical handpiece.
It is also highly desirable to provide a new and improved braking circuit and method for such hand tools which will not consume battery power.
It is also highly desirable to provide a new and improved braking circuit and method which has a reduced number of components to increase reliability and reduce cost.
It is also highly desirable to provide a new and improved braking circuit and method which does not require the command speed to be reduced to zero before braking can occur.
It is also highly desirable to provide a new and improved braking circuit and method which does not cause reverse rotation as it comes to a halt.
It is also highly desirable to provide a new and improved braking circuit and method which controls the rate of braking without resorting to pulsing the braking current which can cause undesirable vibration, delayed braking and reverse rotation.
It is also highly desirable to provide a new and improved braking circuit and method which allows the use of full wave as well as half wave commutation.
It is also highly desirable to provide a new and improved method of retarding rotation of the rotor of a electronically commutated direct current motor by sensing the operator commanded motor rotation and shorting the motor output upon the cessation of commanded rotation while continuing electronic commutation of the motor windings, and dissipating as heat in the motor windings the kinetic energy of the motor rotation.
Finally, it is highly desirable to provide a new and improved hand held tool having all of the above features.